Resistive memory cell having a reduced conductive path area

ABSTRACT

A method of forming a resistive memory cell, e.g., a CBRAM or ReRAM, may include forming a bottom electrode layer, oxidizing an exposed region of the bottom electrode layer to form an oxide region, removing a region of the bottom electrode layer proximate the oxide region, thereby forming a bottom electrode having a pointed tip region adjacent the oxide region, and forming an electrolyte region and top electrode over at least a portion of the bottom electrode and oxide region, such that the electrolyte region is arranged between the pointed tip region of the bottom electrode and the top electrode, and provides a path for conductive filament or vacancy chain formation from the pointed tip region of the bottom electrode to the top electrode when a voltage bias is applied to the memory cell. A memory cell and memory cell array formed by such method are also disclosed.

TECHNICAL FIELD

The present disclosure relates to resistive memory cells, e.g., conductive bridging random access memory (CBRAM) or resistive random-access memory (ReRAM) cells, having a reduced area for the formation of conductive paths (e.g., conductive filaments or vacancy chains).

BACKGROUND

Resistive memory cells, such as conductive bridging memory (CBRAM) and resistive RAM (ReRAM) cells are a new type of non-volatile memory cells that provide scaling and cost advantages over conventional Flash memory cells. A CBRAM is based on the physical re-location of ions within a solid electrolyte. A CBRAM memory call can be made of two solid metal electrodes, one relatively inert (e.g., tungsten) the other electrochemically active (e.g., silver or copper), with a thin film of the electrolyte between them. The fundamental idea of a CBRAM cell is to create programmable conducting filaments, formed by either single or very few nanometer-scale ions across a normally non-conducting film through the application of a bias voltage across the non-conducting film. The non-conducting film is referred to as the electrolyte since it creates the filament through an oxidation/reduction process much like in a battery. In a ReRAM cell the conduction is through creation of a vacancy chain in an insulator. The creation of the filament/vacancy-chain creates an on-state (high conduction between the electrodes), while the dissolution of the filament/vacancy-chain is by applying a similar polarity with Joule heating current or an opposite polarity but at smaller currents to revert the electrolyte/insulator back to its nonconductive off-state.

A wide range of materials have been demonstrated for possible use in resistive memory cells, both for the electrolyte and the electrodes. One example is the Cu/SiOx based cell in which the Cu is the active metal-source electrode and the SiOx is the electrolyte.

One common problem facing resistive memory cells is the on-state retention, i.e., the ability of the conductive path (filament or vacancy chain) to be stable, especially at the elevated temperatures that the memory parts would typically be qualified to (85 C/125 C).

FIG. 1 shows a conventional CBRAM cell 1A, having a top electrode 10 (e.g., copper) arranged over a bottom electrode 12 (e.g., tungsten), with the electrolyte or middle electrode 14 (e.g., SiO₂) arranged between the top and bottom electrodes. Conductive filaments 18 propagate from the bottom electrode 12 to the top electrode 10 through the electrolyte 14 when a bias voltage is applied to the cell 1A. This structure has various potential limitations or drawbacks. For example, the effective cross-sectional area for filament formation, referred to herein as the effective filament formation area indicated as A_(FF), or alternatively the “confinement zone,” is relatively large and unconfined, making the filament formation area susceptible to extrinsic defects. Also, multi-filament root formation may be likely, due to a relatively large area, which may lead to weaker (less robust) filaments. In general, the larger the ratio between the diameter or width of the effective filament formation area A_(FF) (indicated by “x”) to the filament propagation distance from the bottom electrode 12 to the top electrode 10 (in this case, the thickness of the electrolyte 14, indicated by “y”), the greater the chance of multi-root filament formation. Further, a large electrolyte volume surrounds the filament, which provides diffusion paths for the filament and thus may provide poor retention. Thus, restricting the volume of the electrolyte material in which the conductive path forms may provide a more robust filament due to spatial confinement. The volume of the electrolyte material in which the conductive path forms may be restricted by reducing the area in contact between the bottom electrode 12 and the electrolyte 14.

As used herein, “conductive path” refers a conductive filament (e.g., in a CBRAM cell), vacancy chain (e.g., in an oxygen vacancy based ReRAM cell), or any other type of conductive path for connecting the bottom and top electrodes of a non-volatile memory cell (typically through an electrolyte layer or region arranged between the bottom and top electrodes). As used herein the “electrolyte layer” or “electrolyte region” refers to an electrolyte/insulator/memory layer or region between the bottom and top electrodes through which the conductive path propagates.

FIG. 2 shows certain principles of a CBRAM cell formation. Conductive paths 18 may form and grow laterally, or branch into multiple parallel paths. Further, locations of the conductive paths may change with each program/erase cycle. This may contribute to a marginal switching performance, variability, high-temp retention issues, and/or switching endurance. Restricting switching volume has been shown to benefit the operation. These principles apply to ReRAM and CBRAM cells. A key obstacle for adoption of these technologies is switching uniformity.

FIGS. 3A and 3B show a schematic view and an electron microscope image of an example known bottom electrode configuration 1B for a CBRAM cell (e.g., having a one-transistor, one-resistive memory element (1T1R) architecture). In this example, the bottom electrode 12 is a cylindrical via, e.g., a tungsten-filled via with a Ti/TiN liner. The bottom electrode 12 may provide a relatively large effective filament formation area A_(FF), or confinement zone, of about 30,000 nm², for example, which may lead to one or more of the problems or disadvantages discussed above.

SUMMARY

Some embodiments provide resistive memory cells, e.g., CBRAM or ReRAM cells, that focus the electric field more precisely than in known cells, which may provide more consistent filament formation, thus improving the consistency of programming voltage and cell predictability. For example, some embodiments provide techniques for achieving a highly focused electric field emanating from the bottom electrode of the cell. In some embodiments, the effective cross-sectional area, or “confinement zone,” of the bottom electrode may be reduced in comparison to known resistive memory cells. For example, the confinement zone may be reduced to less than 1,000 nm², less than 100 nm², less than 10 nm², or even less than 1 nm².

According to one embodiment, a method of forming a resistive memory cell, e.g., a CBRAM or ReRAM, may include forming a bottom electrode layer, oxidizing an exposed region of the bottom electrode layer to form an oxide region, removing a region of the bottom electrode layer proximate the oxide region, thereby forming a bottom electrode having a pointed tip region adjacent the oxide region, and forming an electrolyte region and top electrode over at least a portion of the bottom electrode and oxide region, such that the electrolyte region is arranged between the pointed tip region of the bottom electrode and the top electrode, and provides a path for conductive filament or vacancy chain formation from the pointed tip region of the bottom electrode to the top electrode when a voltage bias is applied to the memory cell

According to another embodiment, a method of forming an array of cells, e.g., CBRAM or ReRAM cells, may include: forming a bottom electrode layer on a substrate; oxidizing a plurality of exposed regions of the bottom electrode layer to form a plurality of oxide regions spaced apart from each other; removing regions of the bottom electrode layer between adjacent oxide regions, thereby forming a plurality of bottom electrodes, each bottom electrode having a respective oxide region at an upper side of the bottom electrode and at least one pointed tip region adjacent the respective oxide region; forming an electrolyte layer and a top electrode layer over the plurality of bottom electrodes and respective oxide regions; and removing portions of the electrolyte layer and a top electrode layer to form an electrolyte region and a top electrode on each bottom electrode and respective oxide region, thereby forming an array of cells, each cell including a respective bottom electrode, a respective oxide region, a respective electrolyte region, and a respective top electrode; wherein, for each cell, the respective electrolyte region is arranged between the pointed tip region of the respective bottom electrode and the respective top electrode, thereby providing a path for the formation of at least one conductive filament or vacancy chain from the pointed tip region of the respective bottom electrode to the respective top electrode through the respective electrolyte region.

BRIEF DESCRIPTION OF THE FIGURES

Example embodiments are discussed below with reference to the drawings, in which:

FIG. 1 shows an example conventional CBRAM cell;

FIG. 2 shows certain principles of CBRAM cell formation;

FIGS. 3A and 3B show a schematic view and an electron microscope image of an example known CBRAM cell configuration;

FIGS. 4A-4K illustrate an example method for forming an array of resistive memory cells, e.g., CBRAM or ReRAM cells, according to one embodiment of the present invention;

FIG. 5A illustrates a first example top electrode contact configuration, according to one embodiment; and

FIG. 5B illustrates a second example top electrode contact configuration, according to another embodiment.

DETAILED DESCRIPTION

FIGS. 4A-4K illustrate an example method for forming an array of resistive memory cells, e.g., an array of conductive bridging memory (CBRAM) and resistive RAM (ReRAM) cells, according to one embodiment. As shown in FIG. 4A, a dielectric substrate 100 (e.g., SiO₂) is formed, using any suitable technique. Next, as shown in FIG. 4B, a bottom electrode layer 102 and a hard mask layer 104 are deposited or formed over the dielectric substrate 100. Bottom electrode layer 102 may comprise any suitable conductive material or materials, e.g., polysilicon, doped polysilicon, amorphous silicon, doped amorphous silicon, or any other suitable material, and may be deposited or formed in any suitable manner. Hard mask layer 104 may be formed from any suitable materials (e.g., silicon nitride) and may be deposited or formed in any suitable manner as known in the art.

Next, as shown in FIG. 4C, the hard mask layer 104 is patterned, e.g., by forming and patterning a photoresist layer 106 over the hard mask layer 104, using any suitable photolithography techniques. As shown, certain areas of the hard mask layer 104 are exposed through the patterned photoresist layer 106. Next, as shown in FIG. 4D, an etching process is performed to remove the photoresist layer 106 and portions of the hard mask layer 104 corresponding to the exposed areas shown in FIG. 4C, thereby forming a patterned hard mask 104A having an array of openings 105.

The patterning and etching processes of FIGS. 4C and 4D may be selected such that openings 105 have any desired size and shape. For example, openings 105 may have a circular or oval shaped cross-section (in a plane parallel to the bottom electrode layer 102), thus providing cylindrical or elongated cylindrical openings 105. As another example, openings 105 may have a rectangular or otherwise elongated cross-section (in a plane parallel to the bottom electrode layer 102), thus providing elongated trench-style openings 105. Openings 105 may have any other suitable shapes and sizes.

Next, as shown in FIG. 4E, an oxidation process is performed to oxidize areas of the bottom electrode layer 102 that are exposed through the openings 105 in patterned hard mask 104A, thereby forming a number of spaced-apart oxide regions 110. In some embodiments, each oxide region 110 may have a generally oval, rounded, curved, or otherwise non-orthogonal shape in a cross-section extending perpendicular to the bottom electrode layer 102 (i.e., the cross-section shown in FIG. 4E).

Next, as shown in FIG. 4F, the hard mask 104A is removed and the remaining bottom electrode layer 102 and oxide regions 110 are etched to form an array of spaced-apart bottom electrodes 102A and corresponding oxide regions 110. Alternatively, the hard mask 104A may be removed during the etching of the bottom electrodes 102A. The bottom electrode layer 102 and oxide regions 110 may be etched in any suitable manner, e.g., by applying and utilizing a patterned mask or photoresist above the stack, or by using the oxide regions 110 themselves as a mask (e.g., using an etch selective to the non-oxidized bottom electrode material). The etch may or may not be patterned to follow the pattern defined by openings 105 (and thus the pattern of oxide regions 110). Thus, bottom electrodes 102A may have any shape and size, which may or may not correspond with the shapes and sizes of the openings 105 and oxide regions 110 prior to the etch process. For example, bottom electrodes 102A may have a cylindrical or elongated cylindrical shape having a circular or oval shaped perimeter, or an rectangular prism shape have an elongated rectangular perimeter.

In addition, the lateral edges of the etch may be selected with respect to the lateral or outer perimeter edge or extent of each oxide region 110. For example, with reference to FIG. 4E, the lateral edges of the etch may align with the outer perimeter edge of each oxide region 110, as indicated by dashed lines E₁. Alternatively, the lateral edges of the etch may be aligned outside the outer perimeter edge of each oxide region 110, as indicated by dashed lines E₂, such that the post-etch bottom electrode 102A has a region laterally outside the outer perimeter edge of the oxide region 110. Alternatively, the lateral edges of the etch may be aligned inside the outer perimeter edge of each oxide region 110, as indicated by dashed lines E₃, such that the etch extends removes an outer portion of the oxide region 110.

Returning to FIG. 4F, each bottom electrode 102A has a pointed tip region 114 adjacent the respective oxide region. The shape of the pointed tip region 114 may be at least partially defined by the oxide region 110. For example, where the vertical cross-section of the oxide region 110 is oval shaped or otherwise curves downwardly toward the substrate 100, the curved area toward the lateral perimeter of the oxide region 110 helps define the shape of the pointed tip region 114 of the bottom electrode 102A. Thus, in the vertical plane, the pointed tip region 114 may define an angle α of less than 90 degrees, as shown in FIG. 4F.

The pointed tip region 114 may extend partially or fully around the lateral perimeter of the bottom electrode 102A (e.g., a circular, oval, or rectangular perimeter). In some embodiments, the lateral perimeter of the bottom electrode 102A defines a plurality of sides (e.g., a rectangular perimeter defining four sides), and the pointed tip region 114 extends along one, two, three, or more of the perimeter sides.

Next, as shown in FIG. 4G, an electrolyte layer 120 and a top electrode layer 122 are formed over the array of bottom electrode 102A and corresponding oxide regions 110. Electrolyte layer 120 may comprise any suitable dielectric or memristive type material or materials, for example, SiO_(x) (e.g., SiO₂), GeS, CuS, TaO_(x), TiO₂, Ge₂Sb₂Te₅, GdO, HfO, CuO, Al₂O₃, or any other suitable material. Top electrode layer 122 may comprise any suitable conductive material or materials, e.g., Ag, Al, Cu, Ta, TaN, Ti, TiN, Al, W or any other suitable material, and may be deposited or formed in any suitable manner.

Next, as shown in FIG. 4H, the stack is patterned, e.g., by forming and patterning a photomask 130 over the top electrode layer 122, using any suitable photolithography techniques. As shown, certain areas of the top electrode layer 122 are exposed through the patterned photomask 130. In the illustrated embodiment, the patterned photoresist layer 130 covers only a portion of each underlying bottom electrode 102A/oxide region 110.

Next, as shown in FIG. 4I, an etching process is performed to remove exposed portions of the top electrode layer 122 and electrolyte layer 120. In some embodiments, the etch may be selective with respect to the oxide region 110 such that the oxide region 110 and underlying bottom electrode 102A are not removed, while exposing surfaces of the oxide region 110 and bottom electrode 102A. As shown, the remaining portions of the top electrode layer 122 and electrolyte layer 120 define a respective top electrode 122A and electrolyte region 120A for each bottom electrode 102A/oxide region 110 structure.

Next, as shown in FIG. 4J, any remaining portions of the photomask 130 may be removed, leaving an array 138 of resistive memory cells 140. Each cell 140 includes a bottom electrode 102A having an oxide region 110 at an upper surface, a top electrode 122A, and an electrolyte region 120A arranged between the bottom electrode 102A and top electrode 122A. A close-up of one cell 140 is shown in FIG. 4K. As shown, the electrolyte region 120A is arranged between the pointed tip region 114 of the bottom electrode 102A and the top electrode 122A, which provides a conductive path for the formation of conductive filament(s) or vacancy chain(s) from the pointed tip region 114 of the bottom electrode 102A to the top electrode 122A through the electrolyte region 120A, said conductive path indicated by the illustrated dashed arrow CP.

The structure of cells 140, including the pointed tip region 114, may provide a relatively small, or confined, effective filament formation area A_(FF), or confinement zone. For example, the effective filament formation area A_(FF), measured in a plane generally perpendicular to the direction of filament propagation, may be less than 1,000 nm². In some embodiments, the effective filament formation area A_(FF) is less than 100 nm². In particular embodiments, the effective filament formation area A_(FF) is less than 10 nm², or even less than 1 nm². This reduced confinement zone may provide resistive memory cells (e.g., CBRAM or ReRAM cells) with more predictable and reliable filament formation, as compared with cells having a larger confinement zone. This may provide one or more of the following benefits: lower erase current, narrower distribution of low-resistance state (LRS), higher on/off ratio (HRS/LRS), and improved failure rates.

Top electrodes 122A may be connected in or to any suitable circuitry using any suitable contact scheme. For example, FIGS. 5A and 5B illustrate two example schemes for contacting top electrodes 122A. First, as shown in FIG. 5A, top contacts 150 may be formed such that they contact an upper portion of each top electrode 122A, above the respective bottom electrode 102A/oxide region 110. Second, as shown in FIG. 5B, top contacts 150 may be formed such that they contact a lower portion of each top electrode 122A at a location lateral to the respective bottom electrode 102A/oxide region 110. Top contacts 150 may be arranged in any other suitable manner with respect to top electrodes 122A and other cell components.

In addition, it should be understood that each bottom electrode 102A may be contacted (e.g., for connection to a wordline or bitline) in any suitable or conventional manner. For example, each bottom electrode 102A may be contacted from above by dropping down a contact that is recessed or offset from the memory films. As another example, each bottom electrode 102A may be contacted from below by depositing the bottom electrode layer 102 directly on a salicided active silicon region and then making contact to the active region at the end of a line of bits.

Although the disclosed embodiments are described in detail in the present disclosure, it should be understood that various changes, substitutions and alterations can be made to the embodiments without departing from their spirit and scope. 

The invention claimed is:
 1. A method of forming a resistive memory cell, comprising: forming a bottom electrode layer on a substrate; oxidizing an exposed region of the bottom electrode layer to form an oxide region extending laterally from a first lateral side to a second lateral side; removing a region of the bottom electrode layer proximate the oxide region, thereby forming a bottom electrode having a pointed tip region adjacent the first lateral side of the oxide region and having a shape defined in part by a shape of the first lateral side of the oxide region; forming an electrolyte region and a top electrode over at least a portion of the bottom electrode and the oxide region that was formed prior to forming the pointed tip region of the bottom electrode, such that the electrolyte region is arranged between the top electrode and the pointed tip region of the bottom electrode.
 2. The method according to claim 1, wherein the resistive memory cell is a conductive bridging memory (CBRAM) cell.
 3. The method according to claim 1, wherein the resistive memory cell is a resistive RAM (ReRAM) cell.
 4. The method according to claim 1, wherein the electrolyte region provides a path for the formation of a conductive filament or vacancy chain from the pointed tip region of the bottom electrode to the top electrode.
 5. The method according to claim 1, comprising defining the exposed region of the bottom electrode layer by: forming a mask layer above the bottom electrode layer; and patterning and etching the mask layer to expose a region of the bottom electrode layer.
 6. The method according to claim 1, wherein a cross-section of the oxide region in a plane perpendicular to the bottom electrode layer has a generally oval shape.
 7. The method according to claim 1, wherein the pointed tip region of the bottom electrode extends around a perimeter of the bottom electrode.
 8. The method according to claim 1, wherein: the bottom electrode layer defines an elongated edge at a top side of the bottom electrode; and the pointed tip region of the bottom electrode extends along the elongated edge of the bottom electrode layer.
 9. The method according to claim 1, wherein the electrolyte region and the top electrode are formed by: depositing an electrolyte layer and a top electrode layer over the bottom electrode and the oxide region; etching portions of the electrolyte layer and top electrode layer to form the electrolyte region and the top electrode.
 10. The method according to claim 9, wherein etching the portions of the electrolyte layer and top electrode layer exposes a portion of the oxide region.
 11. The method according to claim 1, wherein removing the region of the bottom electrode layer proximate the oxide region forms a bottom electrode having a first lateral side adjacent the first lateral side of the oxide region, wherein the pointed tip region is formed at the first lateral side of the bottom electrode.
 12. A method of forming an array of cells, comprising: forming a bottom electrode layer on a substrate; oxidizing a plurality of exposed regions of the bottom electrode layer to form a plurality of oxide regions spaced apart from each other, each oxide region extending laterally from a respective first lateral side to a respective second lateral side; removing regions of the bottom electrode layer between adjacent oxide regions, thereby forming a plurality of bottom electrodes, each bottom electrode having a respective oxide region at an upper side of the bottom electrode and at least one pointed tip region adjacent the first lateral side of the respective oxide region and having a shape defined in part by a shape of the first lateral side of the respective oxide region; forming an electrolyte layer and a top electrode layer over the plurality of bottom electrodes and respective oxide regions that were formed prior to forming the pointed tip regions of the bottom electrodes; removing portions of the electrolyte layer and a top electrode layer to form an electrolyte region and a top electrode on each bottom electrode and respective oxide region, thereby forming an array of cells, each cell including a respective bottom electrode, a respective oxide region, a respective electrolyte region, and a respective top electrode; wherein, for each cell, the respective electrolyte region is arranged between the pointed tip region of the respective bottom electrode and the respective top electrode, thereby providing a path for the formation of at least one conductive filament or vacancy chain from the pointed tip region of the respective bottom electrode to the respective top electrode through the respective electrolyte region.
 13. The method according to claim 12, comprising defining the plurality of exposed regions of the bottom electrode layer by: forming a mask layer above the bottom electrode layer; and patterning and etching the mask layer to expose a plurality of spaced-apart regions of the bottom electrode layer.
 14. The method according to claim 12, wherein a cross-section of each oxide region in a plane perpendicular to the bottom electrode layer has a generally oval shape.
 15. The method according to claim 12, wherein a pair of adjacent cells in the array of cells are mirror images of each other about a plane defined between the adjacent cells. 